6.2 Primary Clarification Operations

Settling Mechanics & Role in the Treatment Train

Primary clarification (sedimentation) is the physical unit process following preliminary screening and grit removal. Its primary role is to separate settleable organic solids and floatable fats, oils, and grease (FOG) from the wastewater stream prior to secondary biological treatment.

Degritted Influent ──► [ Primary Clarifier: Type II Settling ] ──► Primary Effluent (To Aeration)
                                 │                     │
                                 ▼                     ▼
                       [ Floating Scum/FOG ]   [ Settled Primary Sludge ]
                       (To Scum Well/Pump)     (4-8% TS To Digestion)

Type II Flocculent Settling Dynamics

Sedimentation in primary clarifiers is governed by Type II Flocculent Settling:

• In dilute suspensions of organic particles, individual particles collide, coalesce, and aggregate into larger flocs during their downward trajectory.
• As floc size and particle mass increase, the particle's terminal settling velocity ($v_s$) increases progressively with settling depth and detention time.
• This contrasts with Type I Discrete Settling (observed in grit chambers), where rigid particles settle at a constant terminal velocity without altering shape, size, or density.

Process Benefits for Secondary Treatment

By physically removing particulate matter before biological processing, primary clarification delivers immense operational and energy benefits:

1. BOD Reduction: Reduces secondary aeration organic loading by 25% to 40%, drastically decreasing blower electrical power requirements.
2. Solids Reduction: Removes 50% to 70% of influent TSS, preventing inert and non-biodegradable solids from occupying secondary aeration volume.
3. Sludge Quality: Produces dense, concentrated primary sludge that is readily biodegradable and generates substantial biomethane gas during anaerobic digestion.

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Hydraulic & Operational Design Parameters

California state certification examinations rigorously test the four fundamental primary clarifier design and operational calculations:

1. Surface Overflow Rate (SOR / Surface Settling Rate)

The Surface Overflow Rate represents the upward hydraulic velocity of liquid leaving the clarifier surface area ($A$). Any particle with a downward settling velocity ($v_s$) equal to or greater than the SOR will be completely captured.

$\text{SOR } (\text{gpd/ft}^2) = \frac{\text{Flow Rate } (\text{gallons/day})}{\text{Surface Area } (\text{ft}^2)}$

$\text{Circular Surface Area} = \frac{\pi \times D^2}{4} = 0.7854 \times D^2 \qquad \text{Rectangular Area} = L \times W$

• Typical Design Ranges:
  • Primary clarifiers treating raw wastewater alone: 800 to 1,200 gpd/ft².
  • Primary clarifiers with Waste Activated Sludge (WAS) co-settling: 600 to 800 gpd/ft² (lower rate required because secondary biological floc is less dense and prone to shearing).
  • Peak Wet Weather Flow (PWWF): 2,000 to 3,000 gpd/ft².

2. Hydraulic Retention Time (HRT / Detention Time)

Hydraulic Retention Time is the theoretical average time wastewater resides within the basin volume ($V$).

$\text{HRT (hours)} = \frac{\text{Basin Volume (gallons)}}{\text{Flow Rate (gallons/day)}} \times 24\text{ hours/day}$

$\text{Basin Volume (gallons)} = \text{Area (ft}^2\text{)} \times \text{Depth (ft)} \times 7.48\text{ gal/ft}^3$

• Operating Window: 1.5 to 2.5 hours at average daily flow (typically designed for 2.0 hours).
  • Too Short (< 1.0 hour): Inadequate flocculation and settling; high TSS and BOD carryover into aeration basins.
  • Too Long (> 3.0 to 4.0 hours): In warm California climates, extended detention depletes dissolved oxygen, creating anaerobic conditions where bacteria ferment organics, generate gases, and cause sludge blankets to float.

3. Weir Overflow Rate (WOR)

Weir Overflow Rate measures the volume of clarifier effluent discharging over each linear foot of effluent launder weir plate.

$\text{WOR } (\text{gpd/ft}) = \frac{\text{Flow Rate } (\text{gallons/day})}{\text{Total Active Weir Length } (\text{linear feet})}$

$\text{Circular Perimeter Weir Length} = \pi \times D$

• Target Thresholds: 10,000 to 15,000 gpd/ft at average design flow (maximum 20,000 gpd/ft at peak wet weather flow). Excessive WOR generates high localized upward approach velocities near effluent troughs, pulling settled solids off the bottom into the effluent launders.

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Typical Treatment Removal Efficiencies

Primary clarification removes particulate and settleable constituents while soluble fractions pass through unaffected:

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Clarifier Configurations: Rectangular vs. Circular Basins

        RECTANGULAR CLARIFIER                           CIRCULAR CLARIFIER
┌─────────────────────────────────────────┐              ┌─────────────────┐
│ Influent ──► [Flight Return] ──► Scum   │              │     (Scum Arm)  │
│ (Inlet)      (Surface Skimming)  Box    │              │       ──► ◯ ◄── │ (Center
│                                  │      │              │    Radial Rakes │  Feed)
│ [Sludge Hopper] ◄── [Floor Flight] ◄──  │              │       ──► ◯ ◄── │
│  (Deep Sump)       (Sludge Scraping)    │              │  [Bottom Hopper]│
└─────────────────────────────────────────┘              └─────────────────┘

1. Rectangular Clarifiers

• Hydraulic Flow: Longitudinal horizontal flow with length-to-width ratios typically between 3:1 and 5:1 (e.g., 150 ft long by 30 ft wide).
• Sludge & Scum Collection:
  • Chain-and-Flight Mechanism: Non-metallic polymeric chains (or heavy cast-iron chains) drive full-width fiberglass or redwood flights along the tank floor toward the influent end at 2 to 4 ft/min ($0.6\text{ to }1.2\text{ m/min}$), pushing settled sludge into deep influent hoppers.
  • Surface Return: On their return pass along the water surface, the flights act as surface skimmers, pushing floating scum and FOG toward a rotating slotted scum pipe or scum beach near the effluent end.
  • Cross Collectors: Cross-collector flights or transverse screw augers in the bottom hopper move sludge laterally to the pump suction intake.

2. Circular Clarifiers (Center-Feed vs. Peripheral-Feed)

• Center-Feed Configuration (Most Common): Raw wastewater enters through a central vertical column into a center energy-dissipating feed well. The feed well directs flow downward and radially outward at low velocity.
• Rotating Bridge & Rake Assembly: A central motor-driven drive rotates radial rake arms at 0.02 to 0.05 rpm (outer tip speed approximately 8 to 12 ft/min). Angled steel plow blades or spiral scrapers push settled sludge toward a central center-bottom sludge sump.
• Scum Collection: A surface skimmer blade attached to the rotating bridge sweeps the surface, pushing floating grease up a hinged scum beach ramp into a scum collection box for flushing to a scum well.

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Operational Components & Hydraulic Control

1. Influent Energy Dissipating Well / Baffle: Quells high inlet kinetic energy, dampens turbulence, and distributes incoming flow evenly across the settling cross-section in laminar streamlines.
2. Scum Baffles: Rigid fiberglass baffles submerged 6 to 12 inches (150 to 300 mm) below the liquid level, installed immediately in front of effluent weirs to physically block floating FOG and plastics from overflowing.
3. Effluent Launders & V-Notch Weirs: Precision-leveled 90° or 60° V-notch weir plates mounted along effluent troughs. Proper leveling ensures uniform peripheral withdrawal, preventing localized high-velocity exit channels.

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Operational Troubleshooting & Common Malfunctions

1. Septic Sludge Blanket Gas Lifting ("Burping" or "Clumping")

• Mechanism: If primary sludge is allowed to accumulate in deep blankets for extended periods, anaerobic bacteria ferment volatile organics, producing methane ($CH_4$), carbon dioxide ($CO_2$), and nitrogen gas ($N_2$). Entrained micro-bubbles reduce floc bulk density, causing large chunks of black, foul-smelling sludge to float to the surface ("burping") and spill over effluent weirs.
• Remedy: Immediately increase sludge pumping frequency and duration to evacuate accumulated sludge and maintain blanket thickness below 2 to 3 feet.

2. Hydraulic Short-Circuiting & Density Stratification

• Short-Circuiting: Occurs when wastewater bypasses the active tank volume and travels from inlet to effluent weir in a fraction of theoretical HRT. Caused by unlevel weir plates, missing inlet baffles, or wind-driven surface currents.
  • Diagnostic: Perform a Rhodamine WT dye tracer study; if dye appears at effluent weirs in less than 30 to 45 minutes, severe short-circuiting is confirmed.
• Density Stratification: Occurs when cold influent plunges under warmer basin water, or warm industrial influent skates across the surface, racing directly to effluent launders. Remedied by installing mid-depth vertical baffles and optimizing inlet feed wells.

3. Mechanical Drive Failures & Sheared Pins

• Clarifier drives are protected by mechanical shear pins or electronic torque limiters. If heavy grit, fallen debris, or thick unpumped sludge binds the rake flights, the pin shears to prevent gearbox destruction. Operators must inspect flight alignment and verify rotation daily.